1. Field of the invention
The present invention mainly relates to an encoder and more specifically, to an encoder which optically detects positional information of a moving body.
2. Description of Related Art
In recent years, as a scan-type encoder, an encoder has been proposed which detects positional information of a scale that moves along with a moving body and has gratings periodically formed along the movement direction, by irradiating the scale with irradiation light modulated on the basis of a given modulation signal and then comparing the reflected light or transmitted light thereof with the modulation signal (U.S. Pat. No. 6,639,686).
Also, as the scan-type encoder, there is a technique of detecting the amount of movement of a moving body on the basis of a change in the intensity of interfering light in an interference state by using a three-grating optical system which separates a laser light emitted from a light source into ±first-order light beams using a first diffraction grating and reversing the directions of the separated ±first-order light beams using a second diffraction grating, thereby forming an interference fringe on a third diffraction grating moving along with the moving body (Japanese Patent Application Publication No. 2007-333722),